THE SPECIAL SENSES. 223 



has been termed the labyrinth (see Fig. 53, c). This 

 cavity is hollowed out in dense bone, and consists of three 

 parts; the vestibule (a), or ante-chamber, which is connected 

 with the other two ; the cochlea (b), or snail's shell ; and the 

 three semicircular canals (c). The manner in which the 

 nerve of hearing is distributed is remarkable, and is pecu- 

 liar to this nerve. In the vestibule and the canals its 

 fibres are spread out over the inner surface, not of the bony 

 cavity but of a membranous bag, which conforms to and 

 partially fills that cavity; and which floats in it, being 

 both filled and surrounded with a clear, limpid fluid. 



101. A singular addition to the mechanism of hearing is 

 observed within this membranous bag of the labyrinth 

 This consists of two small oval ear-stones, and a quantity 

 of fine powder of a calcareous nature, which is called "ar- 

 sand." When examined under the microscope, these sandy 

 particles are seen to lie scattered upon and among the 

 delicate filaments of the auditory nerve; and it is probable, 

 that as the tremulous sound-wave traverses the fluid of the 

 vestibule, the sand rises and falls upon the nerve filaments, 

 and thus intensifies the sonorous impression. 



102. In, the cochlea, or snail's shell, which contains the 

 fluid, but no membrane, the nerve ramifies upon a spiral 

 shelf, which, like the cochlea itself, takes two and a half 

 turns, growing continuously smaller as it winds upward. 

 As many as three thousand nerve fibres of different lengths 

 have been counted therein ; which, it has been thought, 

 form the grand, yet minutely small key-board, upon which 

 strike all the musical tones that are destined to be con- 

 veyed to the brain. The vestibule, it is also supposed, takes 

 cognizance of noise as distinguished from musical sounds; 

 while the office of the semicircular canals is, in part at 

 least, to prevent internal echoes, or reverberations. 



101. Where i? the "ear-sand 11 found 1 Give the theory as to its use. 



102. In the cochlea or snail 1 * shell ? '-Key-board 1 ' in the internal ear? The 

 vestibules ? Semicircular canals *> 



